In a nozzle for use in continuous casting, alumina inclusions in molten steel are apt to attach on a surface of an inner hole of the nozzle and coalesce into large inclusions. Such large alumina inclusions are highly likely to be mixed with a molten steel flow and incorporated in slabs, which will cause defects of the slabs and deteriorate quality thereof. In particular, this alumina build-up phenomenon occurs notably in continuous casting of aluminum-killed steel.
Late years, in connection with the need for strictly controlling the quality of steel products, particularly, high-grade steel for sheet metal etc., a good deal of effort has been made to protect a continuous casting nozzle from alumina build-up.
As one of measures against alumina build-up, there has been known a technique of injecting argon gas from a surface of an inner hole of the nozzle into molten steel to physically protect against alumina build-up. In this technique, if argon gas is injected in an excessive volume, gas bubbles will be incorporated into slabs to form pinholes in the slabs, resulting in defective slabs. In other words, an allowable volume of gas to be injected is limited to a relatively small value, and consequently this technique cannot exactly serve as a satisfactory solution.
There has been known another technique of providing an alumina build-up protecting function to a refractory portion itself constituting a continuous casting nozzle. Specifically, the refractory portion is formed to contain CaO so as to induce a reaction with alumina attached thereon to create a CaO—Al2O3 based low-melting-point compound. However, CaO contained in the refractory portion is likely to cause problems about increase in thermal expansion coefficient of the entire nozzle, causing deterioration in resistance to cracks, and hydration of CaO in the refractory material itself.
With a view to utilizing the above alumina build-up protecting function of the CaO-containing refractory portion while minimizing the adverse effects of the refractory portion, it has been proposed to arrange the refractory portion in a nozzle in such a manner as to define only a surface region adapted to be in contact with molten steel. For example, there has been known an submerged nozzle having a CaO-containing refractory portion arranged to define an inner hole. The CaO-containing refractory portion is arranged to define the inner hole in various manners, for example, by forming the refractory portion in a process of forming a body of an submerged nozzle as a single piece, or by forming only the nozzle body and then forming the refractory portion on a surface of a hole of the nozzle body through a coating or casting process, or by forming the refractory portion as a separate member with an inner hole (hole-defining member) and then attaching the hole-defining member to the nozzle body using mortar.
A CaO-containing material having an alumina build-up protecting function includes lime clinker, dolomite clinker and calcium zirconate.
For example, the following Patent Publication 1 discloses a molten-steel casting nozzle having a molten-steel channel which comprises an inside layer containing 20 to 97 mass % of lime clinker and 3 to 80 mass % of carbonaceous material, and an exposed layer containing 50 to 95 mass % of alumina-based material and 5 to 50 mass % of carbonaceous material. This publication also includes a description that a part of the lime clinker may be substituted with a dolomite clinker, or a calcium-zirconia clinker containing 20 wt % or more of CaO. However, in long-term continuous casting or in casting of molten steel containing a large amount of alumina suspended therein, the above nozzle is likely to cause a problem about increase in wear due to release of a low-melting-point compound created through a reaction between Al2O3 contained in molten steel and CaO contained in the clinker.
The following Patent Publication 2 discloses a continuous casting nozzle comprising a coating layer made of a calcareous material containing 50 to 100 mass % of CaO and formed as a wall of an inner hole of the nozzle at a given thickness. This publication also discloses a coating layer made of 80 mass % of dolomite clinker, and 20 mass % of magnesia clinker. While this coating layer can reduce the wear to some extent, it is necessary to further reduce the wear in view of practical use.
It is commonly believed that the refractory portion arranged to define an inner hole preferably has a thickness of about 1 mm at minimum and about 20 mm at maximum, and a material of the refractory portion preferably has a particle size of 1 mm or less. For example, the following Patent Publication 3 includes a description that an average particle size of the material is preferably set at 44 μm or less to have adequate surface properties.
As above, the refractory portion comprising a CaO-containing material, such as dolomite clinker, can react with alumina attached thereon to create a CaO—Al2O3 based low-melting-point compound which will be released from a surface of the refractory potion by a molten steel flow to exhibit an excellent aluminum build-up protecting function. However, this technique has been hardly put into practical use until now, due to difficulties in ensuring the compatibility with corrosion resistance.
[Patent Publication 1] Japanese Patent Laid-Open Publication No.61-53150[Patent Publication 2] Japanese Patent Laid-Open Publication No.63-132755[Patent Publication 3] Japanese Patent Laid-Open Publication No.05-200508